2x mopre clips under wheel well for fill/outlet lines. Was paranoid about damaging the tank - so tapped through with a tiny drill bit, used it as a depth gauge and then cut the heads of my p-clip bolts to make sure they wouldn't interfere with the tank:

On the filler height you can get down as low as 200mm before it starts getting too awkward - At that height a gasguard filler the right way up touches the ground with the hose. Though it depends how easy you find it getting down to that sort of level of course!

One final thing I need to sort is properly securing the tank in place. It's been pointed out a couple of times that centre of the turret is not a permanent piece of the rest of the wheel well, and is bonded in.

Thing is I'm struggling to visualise a solution - so want people's observations of what is acceptable for a safety inspection. My first thought was in fact to install a pair of sold bars above the tank - to prevent it moving forward/upward in the event of a crash, as below:

Straps would be say 2mm thick, 20mm wide flat steel bar, securely bolted through the vehicle body at both ends - and formed slightly so they sit in to the wheel well - with material between them and the tank to prevent rubbing.

If that's not acceptable, does anyone have any examples of how I might do this from underneath? Would it be a case of fabricating some flat spreaders and using them place of the washers that came with the tank - so that the spread the load betwee the bonded and permanent sections of the wheel well - something like this (red lines indicate the spreaders - one slightly out of shot):

Come to think of it - my preference would be to use something like the spreaders, as the straps option is likely to be a pain, in terms of access for the bolts at the rear of the wheel well (likely obscured by diff & driveshafts.

A 2mm thick piece of steel isn't going to hold a tank which may weigh 70kg or more. It may limit its movement in one plane, but it'll still be resting on the bottom bolt as before. To gauge (sic) what is needed, see if the proposed strap deflects when you put your body weight onto it. A 5-6mm thick piece of angle iron is more likely to be up to the job.

When you go over a hump back bridge or corner sharply (or crash), the weight will multiply up so you need a substantial amount of headroom in the calculation of what it takes to hold one of these tanks.

Central turret may 'only' be bonded in but presumably the spare wheel was secured to this bonded in section and BMW wouldn't want to risk the spare wheel hitting you at the back of the head either... Bonding can be strong, mentioned on another thread that some plane wings are bonded on. Nevertheless, probably a good idea to address any potential lack of tank security.

Could have done that by welding or bolting the central section to the rest of the boot floor before fitting the tank but that wouldn't be the easiest way.

Idea of wide bars would do it, I've done that myself. Maybe a pity it's an Irena tank though, as a central line between bolt holes would foul the pipe tube, with other make tanks the bolt holes are offset (of centre line of pipe hole) so a single bar could be used with both bolt holes going through the same bar thus preventing bars rotating without fouling the pipe tube. Some would reckon the bars could rotate into a position that allowed (one of) them to easier pull through the centre section in a crash depending on how long they are.. but a problem with longer bars is that as bolts are tightened they'll tend to be pried away from the boot floor progressively towards the ends (as the area around the bolt is pulled up the ends of the bars go down). To get around both problems I've used shorter bars than I might otherwise have used but welded cross bars onto them to make a window frame shape, the cross bars being the closest to the boot floor while outside of the central bonded area... no bars sticking down at the edges, clamping force is on strong (none middle) section of floor, total area of bars including the 2 additional bars is as much as there'd be with 2 discreet longer bars. Just adjusting the frame design a bit by using longer cross bars, spreading area could be increased with one H shape above another H (a single double H bracket) but would need to make sure edges of H's didn't begin to pry down.

Wouldn't go with the top straps/bars idea, would prevent the tank flying into the car but not prevent the tank flying off from below the car... aka loose battery check during an MOT - tank would of course be trapped so we know it wouldn't go anywhere but so too would a loose battery... some would pick fault with that (not me). If tank had lugs on the top which bars attached to then would be a different matter. Even without top lugs, top bars plus current method of fastening should be enough to convince anyone of security, though may not convince the most pedantic COP11 checker.

Some cars (Mercs) have completely plastic wheel wells, considered strong enough by Merc to prevent the spare flying around in a crash but to be sure tank won't fly around we must fasten it securely to metal. In this case it's better to fit a tank with top lugs and use overbar type securing, else a strong bracket must be made to run from 30degree tank mounting bolts (underneath) to a strong metal part of the car. In the latter case, the rear of all cars has strong metal and if the owner decides they'd like a rear mounted filler ('No holes in the bodywork of my Merc thanks, I'd have it in the plastic bumper if there was an available location but you say there's nowhere with enough clearance between the bumper and floorpan without having to remove some bumper strengthening structure.. Reckon I'll have the filler at the rear then and at least I'll be able to fill from a pump at either side of the car) then same bracket can be extended to also serve as the filler mounting. With the Irena tank the main part of such bracket might be shaped like a tuning fork, open ends clear the pipe hole, long bit extends rearwards.

Mine and Marek's posts crossed... Agreed with Marek, though worth saying that a big enough area of 2mm steel would support the tank in a crash... Marek's point will refer to the idea of 2mm thick narrow bars running over the tank, most 30degree toroidal tanks do bolt to similarly thin steel (wheel well floor) but that's where the spreaders come in, I was talking spreaders made of thicker bar.

Any form of angle iron is much stronger than a flat bar. If space is a problem, start with an angle iron piece and cut 90% of one of the halves off. This still leaves you a structure with rigidity which the flat bar never had.

Any form of angle iron is much stronger than a flat bar. If space is a problem, start with an angle iron piece and cut 90% of one of the halves off. This still leaves you a structure with rigidity which the flat bar never had.

kind regardsMarek

Agreed, Though... Can buy angle that is wider on one side than the other to save the cutting (but limited sizes available). Such angle would need to be described with 3 numbers... thickness of steel, width at one side, width at the other side, length not important to us because all available in 6m lengths, far longer than we need. If we're talking about using angle that's say 3mm thick with 30mm width in parallel with the tank and (in your example) 6mm total sticking up from the tank (cutting 30mm protrusion down to 3mm protrusion along say a metre length if we couldn't buy 3X30X3, which we can't...), then why not use a piece of 6mm flat bar instead? Same raise in height of boot floor but have greater overall strength... we wouldn't want the bars to snap or bolts to pull through one side of it either (bolts pulling through more likely than snap). Reckon 6x40 flat bar would be stronger than (another example angle dimensions) 4x40x8 angle while boot floor could be 2mm lower (+6 instead of +8), in terms of cross sectional area 6x40 = 240sq mm, 4x40x8 = 160sq mm + 16sq mm = 176 sq mm? Pun - or maybe as broad as it is long

I'm having a hard time visualising these solutions in my mind, and am wondering if anyone has any photos of solutions to this issue for BMW E38/E39 cars? Copious googling shows only that single hole toroidal tanks can be used - but not how people solve the problem of re-inforcing - either of the boot floor - or using fabrication from underneath. Anyone got any photos they can share, as I can't find anything useful in Google.

EDIT: Spotted Si's bit above - yes, in original configuration, the full size spare wheel is bolted on to the turret with a wing nut for safe keeping.

20x4mm (or 6mm dependent on locating longer bolts for the tank) flat steel bar spreaders, each with one hole - for the existing tank bolts, aligned to cross the wheel well from left to right if viewed from behind the car, so that the bars sit on each ridge that's part of the permanent wheel well section.

Maybe help with visualising 'window frame' - On your last pic, imagine your bars in same position and then add 2 bars at 90degrees to them, connecting your bars across the top and the bottom of your pic.

The supposedly weak middle turret sticks down a bit from the level of the rest of the boot floor, if the 2 bars we just added were outside the area of that turret and were sandwiched between your original 2 bars and the boot floor they might (they would if they were thick enough) clamp onto the stronger area of the boot floor.

That'll do it! My concerns were that the long lengths of bar would become proud of the wheel well especially towards the ends as you tightened the bolts up, thus my suggestion of using shorter bars and welding cross bars to them (above them), but seems that hasn't been a problem. Not really an issue but I'd generally cut the bolts down a bit depending on how far they stick down.

Just a quick report after around 500 miles and a good few tanks of gas and just under 600 odd miles of running.

I had a coolant leak from the rear coolant hoses that feed the reducer. I had used T peices that were of too small a diameter - and the clamping force on the jubilee cip must have been too high in order to achieve a seal and the hoses split around the clips. I've replaced the hoses and used a wider diameter T piece with a different design of jubilee clip and all is well.

Also serviced the liquid and gas phase filters - quite easy acccess if you pop the front headlight:

Slightly different design of liquid phase filter (old one on the right):

Prins service kit includes both the gas and liquid phase filters and replacement o-rings:

An afternoon well spent. The click/clack of the injectors is less audible now the filters have been changed.

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